Magnetic resonance imaging systems are well known as shown, for example, in U.S. Pat. Nos. 4,830,102 Riederer, 4,568,880--Sugimoto, and 4,558,278--Young. well known as shown in U.S. Pat. No. 4,832,037 Granot. Motion artifact correction schemes for reducing motion artifact from naturally occurring physiologic motion are well known which include monitoring of periodic respiratory or cardiovascular motion, and using the information to synchronize the duty cycle of the NMR apparatus with the motion Also, the order of the phase encoding may be arranged to reduce motion artifacts. Motion artifact correction schemes of the above-mentioned type which include use of a monitor signal are disclosed in U.S. Pat. No. 4,724,386. Method and apparatus for synchronizing operation of the NMR pulse sequence with the cardiac cycle are shown in U.S. Pat. No. 4,903,704 by Van Eggermond et al.
With prior art arrangements, it is possible to obtain images of, say, the heart at various points in the cardiac cycle by use of electrocardiogram gating of the magnetic resonance imaging (MRI) apparatus. A motion picture display of the heart is provided using images obtained at the different points in the cardiac cycle. Such techniques rely upon the existence of a natural physiologic signal, or natural periodic motion from which a timing signal may be derived, to obtain images of the moving object, and are not applicable to imaging of body parts not subjected to involuntary periodic motion.
Also, simulated motion of a joint is well known wherein a plurality of stationary NMR images of the joint are acquired with the joint at different spatial positions, each image being obtained while the joint is stationary. The images then are displayed in succession at a rapid rate to simulate motion of the joint. However, such techniques do not capture dynamic information obtained during joint movement, and do not provide an indication of changes which may occur over the course of time as a result of continued movement of the joint.
Prior art dynamic MRI has been limited to use with timing signals derived from naturally occurring physiologic signals, or motion, of the patient. This greatly limits the application of the MRI technique. For example, to obtain images of a moving appendage, joint, or articulation, there is no naturally occurring physiologic signal that may be employed to gate the NMR system. If there is clinical information or pathology that is observable only during motion, it is important to obtain imaging and/or spectroscopic information during motion for proper diagnosis. In brief, in the absence of naturally occurring physiologic signals to gate the NMR system, prior art MRI and MRS techniques are incapable of capturing dynamic information resulting from recurrent voluntary, or induced voluntary, motion of the patient.